1. Field of the Invention
The present invention relates to a method for removing oxide(s) from a germanium (Ge) semiconductor substrate surface.
It also relates to the Ge semiconductor substrate obtainable by a method according to the invention.
2. Description of the Related Art
In microelectronics industry germanium (Ge) wafers are important substrates with technological applications in optical devices and are very recently introduced as a replacement for silicon (Si) substrates for advanced Integrated Circuit (IC) devices.
Ge has very attractive advantages such as high mobility and compatibility with high-k and III/V materials. Therefore a fundamental understanding of the chemistry of treating Ge-wafers in the various stages of the device production is needed.
Special attention goes to Ge substrate surface preparation where a solution of the problem of how to remove the GeOx completely from the substrate surface must be found.
Making Ge substrate surface oxide free is of particular importance, for example in case of subsequent epitaxial growth where an interface between the Ge substrate and the epitaxial layer must be avoided.
In literature various treatments are described pretending preparing a perfectly oxide free surface. Various concentrations and durations of HCl and HF dips, eventually cycled with a water rinse, has been suggested.
In “Structure of the Cl-passivated Ge(111) surface determined using X-ray absorption and first principles calculations, Surf. Sci., 44, L948 (1999)”, Z. H. Lu et al. teach a method of making a Ge substrate oxide free by means of an HCl(38%):H2O=1:1 mixture.
Deegan and Hughes report a cyclic 50% HF etch followed water rinse procedure (An XPS study of the HF etching of native oxides on Ge(111) and Ge(100) surfaces, Appl. Surf. Sci., 123/124, 66 (1998)).
In “Carbon contamination free Ge(100) surface cleaning for MBE., Appl. Surf. Sci., 125, 125 (1998)”, H. Okumura et al. teach e.g. 2.5% HF, 49% HF and HCl(36%):H2O=1:4.
Our experiments with these mixtures, however, have never confirmed the complete absence of oxides on the Ge wafer surface. In all cases a thin Ge sub-oxide was observed by X-ray photoelectron spectroscopy (XPS) measurement of the Ge wafer surface.